Persistent changes in tree mortality rates can alter forest structure,
composition, and ecosystem services such as carbon sequestration. Our analyses
of longitudinal data from unmanaged old forests in the western United States
showed that background (noncatastrophic) mortality rates have increased rapidly
in recent decades, with doubling periods ranging from 17 to 29 years among
regions. Increases were also pervasive across elevations, tree sizes, dominant
genera, and past fire histories. Forest density and basal area declined
slightly, which suggests that increasing mortality was not caused by endogenous
increases in competition. Because mortality increased in small trees, the
overall increase in mortality rates cannot be attributed solely to aging of
large trees. Regional warming and consequent increases in water deficits are
likely contributors to the increases in tree mortality rates.